Hydrogen embrittlement severely impacts structural materials such as iron, steel, and other metallic alloys. To provide solutions that address the detrimental effect of H, it is necessary to understand the failure mechanisms at the atomic level. One of the proposed mechanisms is hydrogen enhanced decohesion (HEDE), where H accumulation between cleavage planes is expected to reduce the interplanar cohesion. HEDE is expected to occur at grain boundaries (GBs), where H segregation causes intergranular fracture. In ferritic steel GBs, C segregation can add further complexity to the hydrogen embrittlement process. 

In this work, spin-polarized DFT calculations are carried out to investigate the influence of H and C on the decohesion of the Σ5(310)[001] and Σ3(112)[1-10] symmetrical tilt GBs in bcc Fe. A systematic workflow is proposed, addressing specific technical challenges of the first-principles study of the decohesion process of segregated interfaces. The calculated solubility of H at the GB indicates that the more open local environment at the Σ5 GB allows a higher local concentration of H than at the Σ3 GB. In the Σ5 GB, C increases the strength, whereas H can lead to a significant reduction, up to 60%. In contrast, in the Σ3 GB the effect of both elements is limited. These findings suggest that GBs with a more open local atomic environment are more susceptible to H than close-packed GBs. Ultimately, the effect of H on the cohesive strength depends also on the chemical potential, which can in turn vary with the local strain and the presence of alloying elements. The present work thus provides an comprehensive atomistic insight into the HEDE phenomena in ferritic steel GBs.